This invention relates in general to circuitry useful in conjunction with Wheatstone Bridge measuring systems and more particularly, to an apparatus and method for measuring oxygen concentration in gas mixtures by magnetic means without incurring a substantial error due to paramagnetic or thermal properties of background gases.
Accurate measurement of oxygen concentration in a gas mixture is important in a wide range of industrial, clinical and laboratory processes. In response to this need, a variety of devices have been proposed or developed for measuring oxygen concentration. A characteristic of oxygen which is often exploited in these devices is that oxygen is paramagnetic, in that its molecules seek the strongest part of a magnetic field. Most other gases, in contrast, are diamagnetic, in that their molecules seek the weakest part of a magnetic field. This characteristic of oxygen has led to a number of methods and devices for measuring oxygen concentration in gas mixtures.
One type of apparatus for measuring the concentration of oxygen relies upon the inverse relationship between temperature and the magnetic susceptibility of oxygen. As a result of this inverse relationship, heating a portion of an oxygen-containing mixture in a non-homogenous magnetic field creates a "magnetic wind" effect, which can be measured through its thermal effect on an electrical resistance element. Various configurations of magnetic wind devices are discussed in Medlock, et al., "Oxygen Analysis," Transactions of the Instruments and Methods Conference, Stockholm, 1949, pp. 1-8; and Ellis, et al., "The Measurement of Gaseous Oxygen Tension Utilizing Paramagnetism," 40 British Journal of Anesthesia 569 (1968).
Conventional magnetic wind oxygen measurement devices, however, are subject to relatively large errors due to the changes in the thermal properties of the surrounding or "background" gases. In particular, the presence of different background gases causes conventional magnetic wind oxygen sensors to yield false readings of oxygen levels, due to the large differences in thermal characteristics of the background gases.
One type of measuring device, such as that described in U.S. Pat. No. 4,893,495, reduces these effects by nulling the contribution of a single background gas. In this device the measuring circuit is a Wheatstone bridge, which is zeroed for one background condition. In situations where the gas mixture has two or more background gases, this circuitry cannot be zero compensated for the additional background gas.
If, for example, a measurement of oxygen is required over a 0 to 1% range, in a gas stream consisting of nitrogen and oxygen in a conventional oxygen analyzer, the measuring bridge will be adjusted to yield a zero oxygen reading in the presence of 100% nitrogen. The conventional magnetic wind oxygen sensor will then provide correct oxygen measurements, within the limits of the overall analyzer accuracy, of any oxygen concentration within the measuring range of 0-1% oxygen.
If, however, nitrogen is replaced by propane, for example, without resetting the zero compensation, an error would result that is approximately 86% of scale. Although the error for other gases may not be of such magnitude, it can often be greater than tolerable. It will be understood then, that where the background gas mixture includes two or more gases and only one can have zero calibration, this type of error can result.
False readings of this type are not only inconvenient but also may be dangerous. A measuring device showing a falsely low oxygen content may fail to warn of a potentially explosive situation.
Accordingly, there exists a need for circuitry which can provide accurate and reliable measurement of one variable component in a medium, regardless of the concentration of background components of the medium.
It is another object of the invention to provide oxygen sensing methods and apparatus which yield accurate measurement of oxygen concentration, independent of background gas composition and thermal properties.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.